Ignition circuit for inverse-parallel connected ignitrons utilizing a single semiconductor controlled rectifier



1968 H. J. G. M VAN DAELEN 3,3

IGNITION CIRCUIT FOR INVERSE-PARALLEL CONNECTED IGNITRONS UTILIZING ASINGLE SEMICONDUCTOR CONTROLLED RECTIFIER Filed Sept. 1, 1964 INVENTOR.

HENRI J.G.M. VAN DAELEN BY I? AGENT E United States Patent Ofifice3,363,168 Patented Jan. 9, 1968 3,363,168 IGNITION CIRCUIT FURINVERSE-PARALLEL CONNECTED IGNITRONS UTILIZINQ A SINGLE SEMICONDUCTORCONTRGLLED RECTIFIER Henri Jean Gerard Marie Van Daelen, Emmasingel,Eindhoven, Netherlands, assignor to North American Philips Company,Inc., New York, N.Y., a corporation of Delaware Filed Sept. 1, 1964,Ser. No. 393,552 Claims priority, application Netherlands, Sept. 17,1963, 298,023 8 Claims. (Cl. Slit-2d) ABSTRACT OF THE DESCLOSURE Acircuit for supplying power to a load includes two inverse-parallelconnected ignitrons in series with a source of AC voltage and the load.A first center-tapped winding is connected between the anodes of theignitrons. A second center-tapped winding is connected to the anodes andignitors of the ignitrons by :means of a plurality of diodes. Asemiconductor controlled rectifier is connected between the center tapsof the two windings and is pulsed to control the current fiow in thecircuit. The circuit operates so that the full anode voltage is appliedto the ignitors yet the maximum voltage produced across the controlledrectifier is at most equal to one half of the ignitron anode voltage.

The invention relates to a circuit arrangement for the alternateignition of two anti-parallel connected ignitrons by means of a singlecontrollable element.

Such an arrangement is known from French patent specification 1,282,577,particularly from FIG. 2. In this patent the controllable element is aswitch contact conducting in both directions, for example, the contactof a relay.

The invention has for an object to provide a circuit arrangement inwhich this switch contact is replaced by the main current electrodecircuit of a rectifying semiconductor switch, for example, acontrollable semiconductor rectifier. This gives rise to the difiicultythat, for obtaining a satisfactory operation of the ignitrons within anample current and/or voltage range, it is necessary to have apossibility of applying the full anode voltage of each ignitron to theignition electrode thereof, Whilst the voltage operative across thesemiconductor switch may then be inadmissibly high for said element.

The invention provides a solution of this problem so that the full anodevoltage can be applied to the ignition electrode of each ignitron,whilst the maximum voltage operative across the semiconductor switch isat most equal to one half of said anode voltage.

The circuit arrangement according to the invention is characterized inthat the controllable element is a rectifying semiconductor switch, themain current electrode circuit of which is connected between the centraltappings of the windings of two auto-transformers. The central tappingof the winding which is connected to the anode of the semiconductorswitch is connected between the anodes of the two ignitrons. Each sideof the other winding is connected to the anodes of two diodes, thecathodes of which are respectively connected to the anode and to theignition electrode of one of the two ignitrons.

When phase control of the ignition of each ignitron is not desired, therectifying semiconductor switch may be a power rectifier, for example, ap-n-p-n diode having a Well determined threshold voltage, in series withwhich a mechanical on-oif switch can be connected.

The arrangement according to the invention is, however, particularlysuitable when a phase-control of the ignition is desired. The rectifyingsemiconductor switch is then a controllable semiconductor rectifier.

As described in my copending United States application, Ser. No.395,384, filed Sept. 10, 1964, it is frequently useful and oftennecessary to connect, in series with the rectifying semiconductorswitch, in the circuit between the anode of each ignitron and theignition electrode thereof, an element having a stronglyvoltage-dependent resistance and which only becomes strongly conductingwhen the voltage across it exceeds the anode-cathode arc voltage of eachof the two ignitrons, or one half thereof. This element may be avoltage-dependent resistor (VDR). However, use will preferably be madeof a diode connected in the reverse direction and having a Zener voltageexceeding the anode-cathode arc voltage of each of the two ignitrons, orone half thereof.

The invention will be described more fully with reference to thedrawing, which shows a diagram of an embodiment of the circuitarrangement according to the invention.

The embodiment shown comprises two anti-parallel connected ignitrons 1and 2, through which a load 3, for example, a welding transformer, isenergized by an alternating-voltage source 4, for example, an A.C.supply of 50 c./s. The circuit for igniting the ignitrons 1 and 2includes a rectifying semiconductor switch 5, formed by a controllablen-p-n-p-type semiconductor rectifier, the main current electrode circuitof which is connected in series with a semiconductor diode 6, connectedin the reverse direction, between the central tappings of the windings 7and 8 of two autotransformers 9 and 10. The anode of the controllablerectifier 5 is connected to the anode of the diode 6 and the cathode ofsaid diode is connected to the central tapping of the winding 7. Winding7 is connected between the anodes of the two ignitrons 1 and 2.. Then-type emitter of the controllable rectifier 5 is connected to thecentral tapping of the other winding 8 and to a terminal of a source 11of phase-controllable control-pulses having a repetition frequency of,for example, c./s. The control-electrode of the controllable rectifier 5is connected to the other terminal of said source so that positivecontrol-pulses are applied to said control-electrode. Each end of theother winding 8 is connected to the anodes of two diodes 12, 14 and 13,15, respectively. The cathodes of these diodes are respectivelyconnected to the anode and to the ignition electrode of the associatedignitrons 1 and 2, respectively.

If, for example, the upper terminal of the voltage source 4 of, forexample, 220 v. becomes positive, the ignitron 2 is stillnon-conducting, whereas the ignitron 1 is already extinguished becausethe passing current has become lower than its holding current. Thevoltage of the source 4 is therefore operative across the seriescombination of the load 3 and the winding 7. The impedance of saidWinding is high as compared with that of the load 3, so thatsubstantially the whole voltage of the source 4 is operative across thewinding 7.

At the chosen instant, when a forward pulse is applied by the source 11between emitter and control-electrode of the controllable rectifier 5,this rectifier is rendered conducting and the voltage across theleft-hand half of the winding 7 produces a current through this half ofsaid winding, the diode 6, the controllable rectifier 5, the lefthandhalf of the winding 8 and the diode 12. Across the whole winding 8 thiscurrent produces a voltage which is substantially equal to the voltageacross the whole Winding 7 and to the instantaneous value of the voltageof the source 4. This voltage is applied to the ignition electrode ofthe ignitron 2 through the diode 15. As a result of said voltage, an arcis struck between ignition electrode and cathode of the ignitron 2. Thisare forms a cathode spot on the surface of the mercury pool so that themercury vapour in the ignitron is ionised and an arc is struck betweenits anode and cathode. This are carries the main current through theload 3 and forms a strong damping across the winding 7. For theremainder of the half period concerned of the supply voltage, thevoltage across said winding, and hence also across the winding 8, is atmost equal to the anode-cathode arc voltage of the ignitron 2, of, forexample, 20 v.

In accordance with the characteristics of the employed ignitrons, thevoltage of the source 4 and the impedance of the load 3, the dischargebetween ignition electrode and cathode of the ignitron 2 is extinguishedimmediately after the ignition of the main discharge between its anodeand cathode. It may, however, occur that the voltage which is stillapplied between ignition'electrode and cathode after the ignitron of themain discharge is not sufficiently reduced for ensuring the extinctionof the ignition arc. A strong current then continues to fiow through theignition electrode for the remainder of the half period concerned, sothat this electrode finally burns down or is seriously damaged. Acurrent of equal value also continues to pass through the diode 12 andtwice this current passes through the controllable semiconductorrectifier 5, so that particularly the latter element is soon overloaded.This is prevented by the diode 6, which extinguishes as soon as thevoltage across the left-hand part of the winding 7 becomes smaller thanits Zener break-down voltage of, for example, 12 v. The controllablerectifier 5 is then immediately cut off and any further current supplyto the ignition electrode of the i-gnitron 2 is interrupted.

The semiconductor diode 6 may be replaced by another element having astrongly voltage-dependent resistance and which only becomes highlyconducting when the voltage across it exceeds one half of theanode-cathode arc voltage of each of the two ignitrons, for example, bya so-called voltage-dependent resistor (VDR). Instead of using asemiconductor diode or a VDR, use may be made of two correspondingelements, for example, in the connections between each end of theWinding 7 and the anode of the ignitron 1 or 2, respectively, or in theconnections between each end of the winding 8 and the anodes of thediodes 12 and 14 or 13 and 15, respectively, or in series with thediodes 12 and 13 or with the diodes 14 and 15. In accordance with thepoint in the circuit chosen for these elements, they must have athreshold voltage :for becoming conducting which exceeds one half of theanode-cathode arc voltage of each ignitron, or which exceeds the fullarc voltage. In some cases, for example, with semiconductor diodes orVDRs in series with the diodes 14 and 15, the extinction or the decreasein conductivity of these elements does not necessarily involve theblocking of the controllable semiconductor rectifier. A circulationcurrent may, for example, continue to flow for the remainder of the halfperiod concerned through the left-hand or right-hand halves of thewindings 7 and 8, the diodes 12 or 13 and the controllable rectifier 5.

Finally, it should be noted that if a particularly strong ignition ofthe ignitrons 1 and 2 is desired, it is also possible to step up thevoltage across the winding 8, and hence the voltage applied to theignition, electrode of each ignitron by a ratio of more than 1:2, forexample, for compensating -for the voltage loss across the diode 6 oracross corresponding elements. For this purpose it is sufiicient toconnect the anode of each of the diodes 12 and 13 to a tapping of thewinding 8 lying between the end thereof connected to the diodes 14 and15, respectively, and the central tapping of said winding.

What is claimed is:

1. An i-gnitron ignition circuit comprising, a source of AC voltage, anelectric load, first and second ignitrons each having an anode, cathodeand ignitor electrode, means connecting said first and second ignitronsin inverse-parallel relationship in series with said voltage source andsaid load, first and second center-tapped transformer windings, meansconnecting said first transformer winding between the anodes of saidignitrons, first, second, third and fourth diodes, means connecting saidfirst and second diodes between one side of said second transformerwinding and the ignitor and anode of said first ignitron, respectively,means connecting said third and fourth diodes between the other side ofsaid second transformer winding and the ignitor and anode of said secondignitron, respectively, a semiconductor rectifying element having firstand second electrodes defining a current path therein and a controlelectrode for controlling the current in said path, means connectingsaid semiconductor element bet-ween the center taps of said first andsecond windings, and means for applying control pulses to said controlelectrode in synchronism with said AC voltage so as to ignite saidignitrons during alternate half cycles of said AC voltage.

2. A circuit as defined in claim 1 wherein said first and third diodesare poled to conduct current into their respective ignitor electrodesand wherein said second and fourth diodes are poled to allow current toflow in opposite directions in the corresponding halves of said firstand second transformer windings.

3. A circuit as defined in claim 2 wherein said semiconductor elementcomprises a controlled rectifier.

4. A circuit as defined in claim 1 further comprising a zener diodehaving a zener threshold voltage that exceeds one half of theanode-cathode arc discharge voltage of the ignitrons, and meansconnecting said zener diode in series with the current path of saidsemiconductor rectifying element.

5. A circuit as defined in claim 1 wherein said first and third diodesare connected to opposite end terminals of said second transformerwinding and said second and fourth diode, are symmetrically connected tointermediate taps on said second transformer winding between each endterminal and the center tap.

6. A circuit for coupling a source of AC voltage to an electric loadcomprising, first and second ignitrons each having an anode, cathode andignitor electrode, means connecting said first and second ignitrons ininverseparallel relationship in series with said voltage source and saidload, first and'second center-tapped transformer windings, meansconnecting said first transformer winding between the anodes of saidignitrons, first, second, third and fourth diodes, means connecting theanode electrodes of said first and second diodes to one end of saidsecond winding and the cathode electrodes to said ignitor and anodeelectrodes, respectively, of said first ignitron, means connecting theanode electrodes of said third and fourth diodes to the other end ofsaid second winding and the cathode electrodes to said ignitor and anodeelectrodes, respectively, of said second ignitron, a semiconductorrectifier element having anode, cathode and control electrodes, meansconnecting the anode and cathode of said rectifier element to the centertaps of said first and second windings, respectively, and means forapplying a control voltage to said control electrode at given instantsof time during each half cycle of said AC voltage to initiate currentfiow in said rectifier element.

7. A circuit as defined in claim 6 further comprising a zener diodehaving a zener voltage that is greater than one half of theanode-cathode arc discharge voltage of said ignitrons, means connectingsaid zener diode in series With the anode and cathode of saidsemiconductor rectifier element between the center taps of said firstand second Winding, and wherein said voltage applying means includes asource of voltage pulses having a frequency that is twice the frequencyof said AC voltage.

8. A circuit as defined in claim 6 further comprising avoltage-dependent resistor connected in series with said rectifierelement.

References Cited FOREIGN PATENTS 1,031,148 5/1966 Great Britain.

ROBERT SEGAL, Primary Examiner. JAMES W. LAWRENCE, Examiner.

C. R. CAMPBELL, Assistant Examiner.

